Section 8. Scaling and Operational Considerations

Since some of the optional features described above were added to the
   POP3 protocol, experience has accumulated in using them in large-
   scale commercial post office operations where most of the users are
   unrelated to each other.  In these situations and others, users and
   vendors of POP3 clients have discovered that the combination of using
   the UIDL command and not issuing the DELE command can provide a weak
   version of the "maildrop as semi-permanent repository" functionality
   normally associated with IMAP.  Of course the other capabilities of
   IMAP, such as polling an existing connection for newly arrived
   messages and supporting multiple folders on the server, are not
   present in POP3.
   When these facilities are used in this way by casual users, there has
   been a tendency for already-read messages to accumulate on the server
   without bound.  This is clearly an undesirable behavior pattern from
   the standpoint of the server operator.  This situation is aggravated
   by the fact that the limited capabilities of the POP3 do not permit
   efficient handling of maildrops which have hundreds or thousands of
   messages.
   Consequently, it is recommended that operators of large-scale multi-
   user servers, especially ones in which the user's only access to the
   maildrop is via POP3, consider such options as:
   *  Imposing a per-user maildrop storage quota or the like.
      A disadvantage to this option is that accumulation of messages may
      result in the user's inability to receive new ones into the
      maildrop.  Sites which choose this option should be sure to inform
      users of impending or current exhaustion of quota, perhaps by
      inserting an appropriate message into the user's maildrop.
   *  Enforce a site policy regarding mail retention on the server.
      Sites are free to establish local policy regarding the storage and
      retention of messages on the server, both read and unread.  For
      example, a site might delete unread messages from the server after
      60 days and delete read messages after 7 days.  Such message
      deletions are outside the scope of the POP3 protocol and are not
      considered a protocol violation.
      Server operators enforcing message deletion policies should take
      care to make all users aware of the policies in force.
      Clients must not assume that a site policy will automate message
      deletions, and should continue to explicitly delete messages using
      the DELE command when appropriate.
      It should be noted that enforcing site message deletion policies
      may be confusing to the user community, since their POP3 client
      may contain configuration options to leave mail on the server
      which will not in fact be supported by the server.
      One special case of a site policy is that messages may only be
      downloaded once from the server, and are deleted after this has
      been accomplished.  This could be implemented in POP3 server
      software by the following mechanism: "following a POP3 login by a
      client which was ended by a QUIT, delete all messages downloaded
      during the session with the RETR command".  It is important not to
      delete messages in the event of abnormal connection termination
      (ie, if no QUIT was received from the client) because the client
      may not have successfully received or stored the messages.
      Servers implementing a download-and-delete policy may also wish to
      disable or limit the optional TOP command, since it could be used
      as an alternate mechanism to download entire messages.